In the Jan. 24, 1992 issue of Science (Vol. 255, p. 446), applicants Jin and Tiefel describe a class of composite materials which are optically transparent and, at the same time, electrically conductive. These composite materials comprise sheets of polymer containing columns of magnetic conducting spheres.
Such composite materials have a variety of uses due to their anisotropic electrical conductivity. They conduct through the thickness of the material but not laterally. U.S. Pat. No. 4,644,101 issued to Sungho Jin et al. on Feb. 17, 1987 discloses the use of such materials in a pressure-responsive position sensor. The operative principle is that applied pressure forces the spheres through any intervening polymer into contact with one another and through the polymer to the surface. U.S. Pat. No. 5,049,249 shows the use of such material as a means for providing electrical contact between protruding electrical contact regions. The protruding contacts press on the conductive columns to enhance electrical contact.
The present invention is concerned with the magnetic properties of a composite medium rather than its electrical properties, and it is specifically concerned with the use of a composite medium as a material upon which erasable magnetic images can be written and developed.